As described in German patent document 3,344,774, a standard tray for taking a dental impression from a jaw having mobile and immobile gum tissues separated at an action line has a body formed with a U-shaped outer or buccal wall having an edge and a U-shaped inner or lingual wall spaced inwardly from the outer wall and defining therewith a U-shaped cavity adapted to fit over the jaw. The U-shaped cavity is dimensioned to fit with some play over the jaw.
In use the cavity is partially filled with impression material and then is forcibly pressed down on the jaw whose impression is to be taken. The impression material is quite viscous so that, when forced against the gum tissues, it push back the mobile tissues and, as a result, the impression will in effect be of the hard tissues of the jaw. The fit is as close as possible so that the impression material is, in effect, pressurized and forced into intimate contact with the immobile tissues of the jaw.
A common problem with such trays is that the impression material pulls out of them. Thus when the material has hardened sufficiently and the tray is lifted, instead of stripping off the patient, the material pulls out of the tray. Then the impression must be peeled off the patient, a procedure that normally damages it and requires that it be taken again.
One solution to this problem has been the use of special bonding agents in the tray that cause the impression material to stick to the tray. While such a system is in theory quite effective, in reality it has numerous drawbacks. The bonding agent must be applied carefully, and must be made of some safe material as it is being put in the patient's mouth. Furthermore if the finished impression is too solidly bonded to the tray, it is itself damaged on removal.
Accordingly U.S. Pat. No. 4,368,040 of Weissmann proposes a tray with numerous cutouts and ridges that cause the impression to interlock with the tray. This purely mechanical coupling is in part effected by causing the impression material to flow out through the tray. As a result the pressure applied to the material to force it against the tissues of the jaw is substantially reduced and the impression is inaccurate.
German published patent application 3,837,585 describes another system where thin slots are cut through the walls of the tray adjacent their edges. Such a system allows the impression mass to be pressurized and does indeed couple the impression to the tray. Nonetheless the material can still pull rather easily out of the tray. In addition the tray itself does not always form a particularly good fit and seal with the jaw.